reduced DXF string bloat

The DXF importer defined a global std::string constant, only to convert it back to a C string on use.

This commit defines the constant as a C string right away, thus saving 340 B of code and data.

code/AssetLib/DXF/DXFLoader.cpp

@@ -63,11 +63,11 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 using namespace Assimp;
 
 // AutoCAD Binary DXF<CR><LF><SUB><NULL>
-const std::string AI_DXF_BINARY_IDENT = std::string("AutoCAD Binary DXF\r\n\x1a\0");
-const size_t AI_DXF_BINARY_IDENT_LEN = 24u;
+static constexpr char AI_DXF_BINARY_IDENT[] = "AutoCAD Binary DXF\r\n\x1a";
+static constexpr size_t AI_DXF_BINARY_IDENT_LEN = sizeof AI_DXF_BINARY_IDENT;
 
 // default vertex color that all uncolored vertices will receive
-const aiColor4D AI_DXF_DEFAULT_COLOR(aiColor4D(0.6f, 0.6f, 0.6f, 0.6f));
+static const aiColor4D AI_DXF_DEFAULT_COLOR(aiColor4D(0.6f, 0.6f, 0.6f, 0.6f));
 
 // color indices for DXF - 16 are supported, the table is
 // taken directly from the DXF spec.
@@ -156,10 +156,10 @@ void DXFImporter::InternReadFile( const std::string& filename, aiScene* pScene,
     }
 
     // Check whether this is a binary DXF file - we can't read binary DXF files :-(
-    char buff[AI_DXF_BINARY_IDENT_LEN+1] = {0};
+    char buff[AI_DXF_BINARY_IDENT_LEN] = {0};
     file->Read(buff,AI_DXF_BINARY_IDENT_LEN,1);
 
-    if (0 == strncmp(AI_DXF_BINARY_IDENT.c_str(),buff,AI_DXF_BINARY_IDENT_LEN)) {
+    if (0 == memcmp(AI_DXF_BINARY_IDENT,buff,AI_DXF_BINARY_IDENT_LEN)) {
         throw DeadlyImportError("DXF: Binary files are not supported at the moment");
     }
 
@@ -549,7 +549,7 @@ void DXFImporter::ParseEntities(DXF::LineReader& reader, DXF::FileData& output)
         ++reader;
     }
 
-    ASSIMP_LOG_VERBOSE_DEBUG_F( "DXF: got ", block.lines.size()," polylines and ", block.insertions.size(), 
+    ASSIMP_LOG_VERBOSE_DEBUG_F( "DXF: got ", block.lines.size()," polylines and ", block.insertions.size(),
         " inserted blocks in ENTITIES" );
 }